The present invention relates to a method for decoloring liquids, in particular ground water. The present invention furthermore relates to a device for decoloring liquids, in particular ground water, by using such a method, as well as to decolored ground water which has been obtained by using the method according to the present invention.
In practice a number of processes for decoloring ground water exist, namely adsorption on active carbon and membrane filtration. The process of adsorption on active carbon comprises a bed of active carbon, through which the water to be decolored is passed with a residence time of about 10-30 minutes, after which a product flow is obtained from which the colour has been removed. One drawback of such a process is the fact that in practice the filter bed of active carbon is already saturated within a month. The saturated active carbon must then be removed from the filter bed and be returned to the supplier, where the saturated bed material is regenerated by means of steam into active carbon, which can be reused in the filter bed. Since the application of such necessary regeneration operations interferes with a continuous operation, it is usual in practice to connect a number of filter beds in series, so that a continuous operation is ensured. Another drawback of decoloring ground water by using active carbon is the fact that the ground water generally contains certain organic-substances which, if they bond to active carbon, form an ideal seed-bed for bacteria and higher organisms, for example worms. As a result of this luxuriant growth, especially the bacteriological quality of the ground water leaving the filter bed of active carbon will decrease. In addition, the presence of said higher organisms in the distribution network may lead to problems.
Another process for decoloring ground water consists of membrane filtration. The advantage of such membrane filtration on a nano filtration scale (pore size: 0.01-0.001 micron) is the fact that in addition to the removal of the colour, also substances such as iron and manganese can be removed. One drawback of such membrane filtration is the high energy consumption, namely about 0.5 kWh per m3 of purified water. Another major drawback is the fact that 10-20% of the ground water to be decolored leaves the process in the form of a waste flow (concentrate flow). In connection with the possible scarcity of ground in the future it is desirable, therefore, not to waste any ground water.
The object of the present invention is to avoid the aforesaid problems of the known processes for decoloring ground water.
In addition to that it is desirable that a method be developed which is capable of processing, in a continuous manner, ground water of varying composition, in particular of a varying colour intensity, into decolored ground water which also has a high bacteriological quality, that is, which is in principle free from bacteria and viruses.
Another object of the invention is to provide a device for decoloring liquids which requires small capital investments and which enables a high production rate of decolored liquids.
The method as referred to in the introduction is according to the present invention characterized in that the following steps are carried out:
i) subjecting the ground water to a biological treatment in a bioreactor;
ii) feeding the effluent from the bioreactor of step i) to a membrane filtration unit, in which membrane filtration unit a separation between biomass and decolored ground water takes place; followed by
iii) feeding back the biomass from step ii) to the bioreactor;
whereby the ground water is subjected to an oxidation treatment in an oxidation reactor prior to carrying out step i).
Although the term ground water will consistently be used in the description below, it should be understood that besides ground water, also surface water, process water and industrial water can be processed by using the method according to the present invention.
Such a method for biologically purifying waste water, whereby the waste water to be purified is biologically treated in a reactor and the effluent from the reactor is fed to a membrane unit connected after said reactor, is known per se from German Offenlegungsschrift 195 27 295. According to the method described therein, a propellant is fed to the membrane unit, as a result of which the concentrate is fed back to the reactor and the filtrate is withdrawn from the membrane unit by means of a vacuum. Such a method requires a complicated plant consisting of a reactor, a membrane filtration unit, a propellant compressor and a vacuum pump. In addition, the plant is unsuitable for processing waste water flows of considerably different compositions.
Although an aerobic biological reactor connected to ultrafilters is known from the article xe2x80x9cMembrane biological reactor system for treatment of oily wastewaterxe2x80x9d, Knobloc et al., Water Environment Research, 66 (1994) March/April, edition 2, Apr. 3, 1994, pages 133-139, it is not known from said publication to decolor liquids, in particular ground water, by using a method wherein an oxidation treatment, a biological treatment and a membrane filtration separation are successively carried out.
By subjecting the ground water to a biological treatment in a bioreactor, the organic substances present in the ground water will be converted into water and carbon dioxide by bacteria, after which the ground water that has been treated in the bioreactor and the bacteria present therein are carried to a membrane filtration unit. The pore size of the membrane filtration unit is selected such that the biomass, that is, the bacteria, do not pass through the membrane, whilst the ground water treated in the bioreactor will diffuse through the membrane of the membrane filtration unit. Then the biomass is withdrawn from the membrane filtration unit and returned to the bioreactor. According to such a method conversion of organic substances takes place in the bioreactor, after which the suspension of biomass and biologically treated ground water is separated into decolored ground water and biomass in the membrane filtration unit. In particular embodiments it is preferred to discharge a part of the amount of biomass to be returned to the bioreactor. Since the liquid to be treated has in principle been freed from bacteria and viruses already by the membrane filtration unit, a liquid having a high biological quality is thus obtained.
The determination of the colour intensity takes place in accordance with Dutch standard NEN 6413, which document may be considered to be incorporated herein. According to this document, the colour intensity of water is indicated by the number of mg of platinum which must be added in the form of potassium chloroplatinate to 1 liter of colourless water in order to give said water the same colour intensity as the water to be examined. A content of less than 5 mg of Pt/Co/1 is indicated as colourless.
The present applicants have carried out extensive research with regard to the colour intensity of water, and they have found that the presence of humic acids or humic acid compounds has a significant influence thereon. Consequently, they propose the following theory, whereby it should be understood that they are by no means bound thereby. Said research has shown that the colour of the ground water is mainly caused by the presence of humic acids or humic acid compounds. The term humic acid is understood to mean organic compounds having complex structures built up of several benzene-like six-membered nuclei having a molecular weight of at least 1000 Dalton or higher. Although the bacteria present in the bioreactor are capable of decomposing such humic acids or humic acid compounds into smaller organic compounds, eventually water and CO2, such a conversion will take place slowly in practice. By first subjecting the ground water to an oxidation treatment in accordance with the method according to the present invention, the humic acids or humic acid compounds present in the ground water, in addition to the other organic compounds, will be decomposed into smaller organic fractions, which fractions will subsequently be converted into water and CO2 relatively quickly in the bioreactor by the biomass that is present. Thus it is possible according to the present invention to decolor liquids, in particular ground water, in an efficient manner.
The oxidation treatment in the oxidation reactor is preferably carried out by using ozone that is injected. It is preferred to capture the unused amount of ozone in the oxidation reactor, and subsequently reintroduce it into the oxidation reactor so as to minimize the amount of ozone that is being used. Although the oxidation treatment is preferably carried out by means of ozone, it is also possible to use another suitable oxidation agent, for example a hydrogen peroxide solution or a permanganate solution, or another oxidation method, for example an enzymatic method. One drawback of using a hydrogen peroxide solution is the fact that it is not possible in practice to feed back hydrogen peroxide solution, and that too high a dose of hydrogen peroxide leads to disadvantageous conditions for the biomass present in the bioreactor.
In another embodiment it is preferred to subject the pumped-up ground water to a treatment for removing iron, manganese, ammonia and constituents which cause the hardness of the liquid to be treated in accordance with a usual method before subjecting the ground water to an oxidation treatment and/or a biological treatment. Such a pre-treatment is only necessary if it is to be expected that iron and/or manganese flocks will be formed during the oxidation treatment in the oxidation reactor, which flocks have an adverse effect on the membrane filtration unit, in particular by clogging the pores.
In a preferred embodiment of the method according to the present invention, the decolored ground water from step ii) is partially returned to the bioreactor. An embodiment such as this one is used in particular if the colour intensity of the ground water that has already been biologically treated is insufficient.
In another preferred embodiment of the method according to the present invention, the decolored ground water from step ii) is partially returned to the oxidation reactor. An embodiment such as this one is used in particular if the colour intensity of the ground water that has already been biologically treated has been reduced to an insufficient degree, for example due to the remaining presence of humic acids or humic acid compounds. When the already insufficiently decolored ground water is subjected to the oxidation treatment in the oxidation reactor anew, the humic acids or humic acid compounds that are still present will be decomposed into smaller organic fractions as yet, which fractions will then be converted in the bioreactor into the compounds water and CO2, which do not cause any colouration.
If it is observed in practice that the colour intensity of the ground water that has been treated already is still undesirably high, it is also possible to increase the residence time in the oxidation reactor and/or in the bioreactor, in order to decrease the colour intensity as yet.
Since a suspension of ground water and biomass is present in the bioreactor, it is desirable to provide a maximum contact area between the biomass and the compounds to be decomposed that are present in the ground water. Said keeping in suspension is preferably carried out by supplying air to the bioreactor, whereby it should be noted, however, that the bioreactor may be provided with a stirring apparatus. The air that is supplied moreover has an advantageous influence on the living conditions of the biomass that is present in the bioreactor. In addition to that it is preferred to create an optimum environment for the bacteria in the bioreactor, so that the conversion into non-colouring organic fractions will take place as quickly as possible. Carboniferous nutrients, in particular acetate compounds, may be fed to the bioreactor, if desired. In order to obtain advantageous conditions for the biomass, it is furthermore preferred for the pH-value in the bioreactor to range from 6-10, in particular from 7-9.
The contents of the bioreactor are then fed to a membrane filtration unit, as described in step ii), whereby the pore size of the membrane is preferably  less than 0.5 micron, in particular  less than 0.005 micron. The function of the membrane is to effect a separation between biomass and treated ground water, whereby the dimension of the biomass determines the critical pore size of the membrane. The dimensions of bacteria generally range from 0.5-3 micron, so that the desired pore size of the membrane is  less than 0.5 micron, preferably  less than 0.005 micron. Another advantage of using a pore size of  less than 0.005 micron is that no ion particles, which may be objectionable, will remain in the decolored ground water.
In another preferred embodiment of the method according to the invention, it is preferred to use a residence time in the oxidation reactor of maximally 600 seconds. A residence time of more than 600 seconds requires a large oxidation reactor or a small volume flow through the oxidation reactor, which aspects are both undesirable in practice.
The residence time in the bioreactor is preferably one hour at most. A residence time in the bioreactor of more than one hour requires the use of large bioreactors and/or low flow rates, which is undesirable in practice.
The temperature of the liquid to be decolored, in particular ground water, is not critical, but it preferably ranges between 0xc2x0 C. and 60xc2x0 C. A temperature below 0xc2x0 C. leads to icing, which renders pumping of the liquid flows impossible, and a temperature above 60xc2x0 C. will cause the bacteria which are present in the bioreactor to die. In practice the ground water that is pumped up will have a temperature of about 10xc2x0 C., and said temperature will not change during the carrying out of the method for decoloring ground water. For particular embodiments it may be desirable, however, to change the temperature of the liquid to be decolored, in particular to the temperature at which the biological conversion in the bioreactor takes place optimally.
The present invention furthermore relates to a device for decoloring liquids, in particular ground water, by using the method according to the present invention. The device according to the present invention, which comprises a bioreactor and a membrane filtration unit connected in series thereto, which device is provided with the necessary supply and discharge pipes and pumps, whereby the membrane filtration unit is also connected to the bioreactor via a return pipe for biomass, is characterized in that an oxidation reactor is disposed before the bioreactor, whereby the outlet of the oxidation reactor is connected to the inlet of the bioreactor.
The oxidation reactor is furthermore preferably provided with a supply pipe for ozone and with a discharge pipe for unused ozone, which discharge pipe is connected to the supply pipe for ozone. In this manner ozone is used optimally. In a particular embodiment of the present invention, it is preferred to have a pre-treatment unit disposed before the oxidation reactor, whereby the outlet of the pre-treatment unit is connected to the inlet of the oxidation reactor. One or more means for removing iron, manganese, ammonia and/or constituents which cause the hardness of the liquid to be treated may be mentioned as suitable pre-treatment units. Such a pre-treatment unit may be provided with return pipes.
In addition to that it is preferred in certain embodiments for a return pipe to be provided in the discharge pipe for decolored ground water from the membrane filtration unit, which return pipe is connected to the bioreactor. According to such a construction, it is possible to feed back the ground water that has not been decolored sufficiently to the bioreactor, whereby the biological treatment in said bioreactor leads to further decomposition of the organic constituents that impart the ground water its colour into the eventual constituents water and carbon dioxide, which do not impart colour.
In certain embodiments it is preferred however, for a return pipe to be provided in the discharge pipe for decolored ground water from the membrane filtration unit, which return pipe is connected to the oxidation reactor. Any ground water that has not been decolored sufficiently is thus returned to the oxidation reactor, in which oxidation reactor the organic constituents that are still present in the ground water are converted into smaller organic constituents, which smaller organic constituents -are then converted into water and carbon dioxide in the bioreactor.
The pore size of the membrane in the device according to the present invention is determined by the size of the biomass to be separated, which biomass generally has a dimension ranging from 0.5-3 micron. The preferred pore size of the membrane is less than 0.5 micron, therefore, in particular less than 0.005 micron.
A suspension of biomass and ground water is present in the bioreactor, and in order to prevent said biomass from settling, the bioreactor is preferably provided with an air supply pipe. It is also possible, however, to provide the bioreactor with a stirring apparatus, whereby it is preferred in some cases to provide the bioreactor with an air supply pipe and with such a stirring apparatus.
Although it is stated in the above description that a preferred embodiment of the device comprises a return pipe for decolored ground water to the bioreactor on the one hand and to the oxidation reactor on the other hand, it is also possible, of course, to split the return pipe for decolored ground water from the membrane filtration unit into a pipe which is connected to the inlet of the bioreactor and a pipe which is connected to the inlet of the oxidation reactor. Such an embodiment is used in particular if the volume of the individual oxidation reactor and that of the individual bioreactor are insufficient for processing the total amount of decolored ground water that is to be returned. The determination of the ratio between the amount to be returned to the bioreactor and the amount to be returned to the oxidation reactor can take place in a simple manner by a person who is skilled in this field.